Class DistinctOnClause

lib/sqlalchemy/dialects/postgresql/ext.py:558–599

class DistinctOnClause(SyntaxExtension, expression.ClauseElement):
    stringify_dialect = "postgresql"
    __visit_name__ = "postgresql_distinct_on"

    _traverse_internals: _TraverseInternalsType = [
        ("_distinct_on", InternalTraversal.dp_clauseelement_tuple),
    ]

    def __init__(self, distinct_on: Sequence[_ColumnExpressionArgument[Any]]):
        self._distinct_on = tuple(
            coercions.expect(roles.ByOfRole, e, apply_propagate_attrs=self)
            for e in distinct_on
        )

    def apply_to_select(self, select_stmt: expression.Select[Any]) -> None:
        if select_stmt._distinct_on:
            raise exc.InvalidRequestError(
                "Cannot mix ``select.ext(distinct_on(...))`` and "
                "``select.distinct(...)``"
            )
        # mark this select as a distinct
        select_stmt.distinct.non_generative(select_stmt)

        select_stmt.apply_syntax_extension_point(
            self._merge_other_distinct, "pre_columns"
        )

    def _merge_other_distinct(
        self, existing: Sequence[elements.ClauseElement]
    ) -> Sequence[elements.ClauseElement]:
        res = []
        to_merge = ()
        for e in existing:
            if isinstance(e, DistinctOnClause):
                to_merge += e._distinct_on
            else:
                res.append(e)
        if to_merge:
            res.append(DistinctOnClause(to_merge + self._distinct_on))
        else:
            res.append(self)
        return res